A possible size-determined directional selection in Danaus plexippus (Lepidoptera: Danaidae) with evidence from stable isotope analysis

Each fall, the North American population of monarch butterflies engage in one of the most extensive insect migrations of up to 4,800 km to their overwintering areas in central Mexico. Since monarchs make extensive use of soaring flight and may have to withstand adverse weather condition during the trip, their wing size may influence survival. Computational Fluid Dynamics analysis illustrated that larger wings produce more lift than smaller wings. The question whether there is directional selection for longer wings during the fall migration in eastern North American monarch butterflies was addressed. This was the first study which used the stable isotope technique to analyze monarch butterflies that had been sampled at several locations of their migration routes during the fall. Hydrogen stable isotope ratios in precipitation show a distinct geographic pattern across the North American continent, which is transferred to the monarch wing at the time and place of formation. This allows the assignment of butterflies to the latitude of their natal origin. The analyses indicate that butterflies with longer wings may have originated from farther north, which means that smaller monarchs may be selected against during the migration. Furthermore, monarchs collected at a location in Texas in 2007 had a larger average wing length and originated from higher latitudes than those sampled at the same location in the previous year. In addition, the general applicability of the stable isotope technique to assign monarchs to the site of their natal origin was addressed. Despite variation in isotope values, the hydrogen stable isotope can be used to determine the latitude of origin.